A study by researchers at Sheffield University suggests that it is noise that is causing the decline in House Sparrows.
Apparently noise in urban areas could be increasing the mortality rate amongst young Sparrows. The researchers found that the noise stopped adult birds from hearing the hunger calls from their chicks.
The UK population declined by an estimated 71% between 1977 and 2008. Numbers are continuing to decline in England, however Scotland and Wales have recorded an increase in recent years.
You can read much more on the research here.